1. Field of the Invention
The present invention is directed to an optical waveguide fiber having a segmented core design. In particular, the core is designed to provide a fiber that reduces non-linear effects while maintaining a standard resistance to bend.
2. Technical Background
A waveguide having large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, all of which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/Aeff, where P is optical power. For example, a non-linear optical effect may be characterized by an equation containing a term, exp [Pxc3x97Leff/Aeff], where Leff is effective length. Thus, an increase in Aeff produces a decrease in the non-linear contribution to the degradation of a light signal. The benefit of large Aeff can also be illustrated using the equation for refractive index that includes the non-linear refractive index. The refractive index of silica based optical waveguide fiber is known to be non-linear with respect to the light electric field. Refractive index may be expressed as,
n=n0+n2 P/Aeff, where n0 is the linear refractive index, n2 is the non-linear index coefficient, P is light power transmitted along the waveguide and Aeff is the effective area of the waveguide fiber. Because n2 is nearly a constant of the material, increasing Aeff is an effective means for reducing the non-linear contribution to the refractive index, thereby reducing the impact of Kerr type non-linearities.
The need in the telecommunication industry for greater information capacity over long distances, without electronic regeneration, continues to encourage investigation of waveguide fiber refractive index profiles that provide enhanced operating properties with regard to non-linear effects and wavelength division multiplexing. In systems that make use of wavelength division multiplexing, a fiber having low dispersion slope is preferred. At the same time, such properties as, attenuation, bend resistance, and fiber strength are expected to be comparable to those of existing waveguide fiber.
One focus of this investigation has been the search for less complex index profile designs that provide the desired performance parameters, but still are reasonably compatibility with a fiber manufacturing environment, so that costs may be controlled.
The present invention is directed to a core refractive index profile species, of the segmented core genus, that reduces non-linear effects and which is particularly suited to transmission of high power, multiplexed signals over long distances without regeneration. The definition of high power and long distance is most meaningful in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impact upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mW. For example, a long distance is one in which the distance between electronic regenerators can be in excess of 100 km.
There is a continuing need for an optical waveguide fiber designed to have the properties similar to those of standard step index fiber or standard dispersion shifted fiber with the additional properties of relatively large effective area and low dispersion slope. The window of operation of greatest interest at this time is that near 1550 nm. The fiber of this invention can be designed to operate over this window, which may extend from, for example, about 1400 nm to 1700 nm.
The following definitions are in accord with common usage in the art. The radii of the segments of the core are defined in terms of the index of refraction of the segment material. A particular segment has a first and a last refractive index point. A central segment has an inner radius of zero because the first point of the segment is on the centerline. The outer radius of the central segment is the radius drawn from the waveguide centerline to the last point of the refractive index of the central segment. For a segment having a first point away from the centerline, the radius from the waveguide centerline to the location of this first refractive index point is the inner radius of that segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point of the segment is the outer radius of that segment.
The segment radius may be conveniently defined in a number of ways. In this application radii are defined in accord with FIG. 1, described in detail below.
The definitions of segment radius and refractive index used to describe refractive index profile in no way limit the invention. Definitions are given herein because in carrying out model calculations, the definitions must be used consistently. The model calculations set forth in the table below are made using the geometrical definitions illustrated in FIG. 1.
The effective area is generally defined as,
xe2x80x83Aeff=2xcfx80(∫E2r dr)2/(∫E4r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The relative index of a segment, xcex94%, as used herein, is defined by the equation,
xcex94%=100xc3x97(nixe2x88x92nc)/nc1
where ni is the maximum refractive index of the index profile segment denoted as i, and nc, the reference refractive index, is taken to be the minimum index of the clad layer. Every point in a segment has an associated relative index. The maximum relative index is used to conveniently characterize a segment whose general shape is known.
The term refractive index profile or simply index profile is the relation between xcex94% or refractive index and radius over a selected segment of the core. The term alpha profile refers to a refractive index profile that may be expressed by the equation,
n(r)=n0(1xe2x88x92xcex94[r/a]xcex1) ,
where r is core radius, xcex94 is defined above, a is the last point in the profile segment, the value of r at the first point of the xcex1-profile is chosen in accord with the location of the first point of the profile segment, and xcex1 is an exponent which defines the profile shape. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding is usually due to dopant diffusion in regions of rapid refractive index change.
Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion is sometimes called chromatic dispersion in the art. The units of total dispersion are ps/nm-km.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. A bend test referenced herein is the pin array bend test that is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven in a serpentine path through the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuation values. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the serpentine woven waveguide fiber conform to the portions of the pin surface at which there is contact between fiber and pin.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the waveguide.
A first aspect of the invention is a single mode optical waveguide fiber having a segmented core region. The core has a central segment and at least one annular segment surrounding the central segment. Each segment is defined in terms of its radii and xcex94%. The central segment is a void that therefore has a refractive index of about 1 over its radius. The relative index of the void is negative because the reference index has been chosen to be silica. A feature of the invention is that the central segment has a large negative relative index without the usual process complications that result from down doping a glass.
The radii and relative indexes of the segments are selected to provide an effective area greater than about 70 xcexcm2 and a dispersion slope less than about 0.08 ps/nm-km. In addition, the induced attenuation under lateral load, as defined above, is less than about 0.8 dB/m.
An embodiment of the waveguide fiber is characterized by a zero dispersion wavelength in the range of about 1450 nm to 1650 nm.
Another embodiment is characterized by a radius of the central void in the range of about 0 to 3 xcexcm.
Yet a further embodiment includes, in addition to the central void, three annular segments that surround the central void. The annular segments are layered one upon another. The central segment and the three annular segments have respective relative indexes, xcex941% greater than xcex943% greater than xcex942% greater than xcex940%, where the segment numbering is consecutive beginning with 0 for the central segment.
A further embodiment has a second annular segment for which the relative index xcex942% is negative.
The index profile shapes of each respective annular segment can have a shape that is a step, and xcex1-profile, a rounded step, or a trapezoid in any permutation or combination.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.